This invention relates to a getter and an electrical switching system using such getter, wherein the getter is made of a porous getter material which has an adsorptive effect on substances which could create resistive films on electrical contacts.
The purpose of using a getter in a sealed contact chamber is to prevent molecules, e.g. of organic compounds, from forming resistive films on electrical contacts by long-termed adsorption. Although it is known in the art that the gettering effect should be selective with respect to such particular substances, previous solutions were insuccessful because it had not been recognized that a generally high gettering effect even can be harmful for electrical contacts. This is true, for example, if conventional getter materials known from vacuum technology are used. The pores of such getter materials have average diameters of no more than 2 to 4 nm and are therefore mainly only capable of adsorbing small molecules, for example those of protective gases present in the contact chamber.
In the periodical "Siemens Components" No. 19 (1981), vol. 5, page 158, there is an indication that, due to variations in the pore size as caused in production, the inner surface of an activated carbon getter can amount to as much as 2.000 m.sup.2 /g which leads to the conclusion that the pores are extremely small, with an average diameter of less than 2 nm. Such a getter will cause a reduction of the pressure which, in turn, will considerably reduce the breakdown voltage of the contacts. On the other hand, if the leakage rate of the contact chamber is greater than 10.sup.-5 cm.sup.3.bar/s, the getter may be saturated by gases leaking into the contact chamber before the relay or other switching device is put into operation. In this case, a gettering of molecules which create resistive films will take place to an insignificant extent only.
It is furthermore known from the same periodical that, with activated carbon getters having widely varying pore diameters, loaded with styrene vapor and produced in accordance with a special technology, a relative increase in weight of about 50% resulting in a saturation of the getter occurs already after 2.5 hours. Since it is further described that the getter has an absorptive power of .DELTA.V=2 cm.sup.3 and a contact chamber having a volume V1=0.12 cm.sup.3 was available, an apparent inner volume V2=V1+.DELTA.V=2.12 cm.sup.3 was available when the getter was introduced in the contact chamber. Under these circumstances, if the contact chamber is sealed at a pressure P1=1 bar=10.sup.5 N/m.sup.2, the pressure P2 within the contact chamber at thermal pressure is reduced to about EQU P2=V1/V2.multidot.P1=0.12/2.12.multidot.1 bar=56.6 mbar,
which causes a decrease in the breakdown voltage by more than 60%.
To avoid reduction in quality by a lowering of the pressure and also to avoid premature saturation of the getter, German patent specification No. 2 462 277 proposes an approach in which a BaOFe magnet having relatively large pores is activated as a getter, with the option of using an additional getter. This turned out to be quite useful over long terms. By using the activated magnet alone, a reduction of the resistance of the films existing on the contacts by about 10 m.OMEGA., thus an increase in the contact reliability by a factor of about 100 was achieved. Nevertheless, foreign layers on the average order of about 10 m.OMEGA. remained. The use of an additional getter having substantially smaller pores led to further success. This, however, has the disadvantage of increased production cost and/or the restriction to polarized relays or other polarized switching devices.
German patent specification No. 1 243 271, German Auslegeschrift No. 2 646 680, and German Offenlegungschrift No. 2 931 596 propose to produce the contact chamber or a coil bobbin forming a contact chamber entirely or partly of a getter material. In one case, it was intended to bind ions generated by discharges within the contact chamber, while the other case aimed at a higher gettering effect than that achieved with known getter materials such as activated carbon. In these proposals, however, the above-mentioned disadvantageous evacuating effect is even stronger, particularly since only the gettering effect but not the adsorptive power with respect to molecules forming resistive films was taken into account. Actually, it would have been necessary to limit the gettering surface, i.e. the gettering effect, to a certain degree to prevent the pressure from being substantially reduced. This would be different in vacuum or high-vacuum contact chambers in which pressures below 10.sup.-1 mbar exist.
It is an object of the invention to provide a getter for use in a sealed contact chamber, which is capable of a long-term and selective adsorption chiefly of those molecules which have a tendency to form resistive films on the contacts, and which can be employed with relays and other switching devices without problem.
It is another object of the invention to provide an electrical switching system which includes electrical contacts and a getter disposed in a housing sealed against the environment, the getter being capable of a long-term and selective adsorption chiefly of those molecules which have a tendency to form resistive films on the contacts, and which can be employed with relays and other switching devices without problem.